# Homework Help: Having trouble starting gravity problem.

1. Oct 9, 2011

1. The problem statement, all variables and given/known data

A. Calculate the earth's gravity force on a 70 kg astronaut who is repairing the Hubble Space Telescope 600 km above the earth's surface

B. Compare this gravity force with his weight at the earth's surface. (F/w)

2. Relevant equations

No idea

w=GmEm/RE^2 is just for the weight, but I don't know how to find part 1, which is needed for part 2.

3. The attempt at a solution

I don't know what formulas to use to answer these questions. Thanks.

Last edited: Oct 9, 2011
2. Oct 9, 2011

### SteamKing

Staff Emeritus
What's a 70 km astronaut look like?

If you can calculate the weight of the astronaut at an altitude of 600 km, then you should easily be able to determine the weight of the same astronaut on the ground.

3. Oct 9, 2011

### Villyer

Is the problem here just knowing how to interpret the formula you have, like you don't know what all of the variables mean and where to plug stuff in?

4. Oct 9, 2011

At height h, g = 9.8 x [R/(R+h)]^2
So find g, then use W=mg. The weight, W, is the gravity force.

problem is I get 9.80 m/s^2 here! So then the weight on the ground is the same.

This makes no sense.

9.8 * [6.38*10^6/(6.38*10^6+600)]^2 = 9.80 m/s^2 at the distance of 600 km.

so weight is 686 N, but so is it at Earth. What am I doing wrong?

Last edited: Oct 9, 2011
5. Oct 9, 2011

Probably, I keep messing the formula I found up miserably.

6. Oct 9, 2011

### Villyer

In the radius at the bottom, you added 600. 600 is kilometers, 6.38*10^6 is meters.

Nevermind about the second point I had here a second ago, I was thinking of a different equation.

7. Oct 9, 2011